Correlation between floppy to rigid transitions and non-Arrhenius conductivity in glasses

Non-Arrhenius behaviour and fast increase of the ionic conductivity is observed for a number of potassium silicate glasses $(1-x)SiO_2-xK_2O$ with potassium oxide concentration larger than a certain value $x=x_c=0.14$. Recovering of Arrhenius behaviour is provided by the annealing that enhances densification. Conductivity furthermore obeys a percolation law with the same critical concentration $x_c$. These various results are the manifestation of the floppy or rigid nature of the network and can be analyzed with constraint theory. They underscore the key role played by network rigidity for the understanding of conduction and saturation effects in glassy electrolytes.Comment: 4 pages, 4 EPS figure

The dynamics of income inequality in Africa: An empirical investigation on the role of macroeconomic and institutional forces

Reducing income inequality is a crucial goal of sustainable development as income inequality often viewed as harmful to economic growth. The main aim of this paper was to empirically assess the macroeconomic and institutional drivers of income inequality in Africa. We use a Kuznets curve framework, which emphasises the role of income per capita in explaining the time path of inequality. In contrast to much of the literature, we explicitly examine the possibility of the existence of multiple income steady states. Using the concept of clubs of convergence, we show that per capita income is divergent and identify four steady states to which groups of economies converge (i.e., high-income to low-income economies). Using panel data models and a data set encompassing 52 African countries spanning the years 1980–2017, we show that once these multiple steady states are accounted for, the Kuznets curve relationship becomes unstable. Our findings suggest that inequality may be increasing in high-income countries in Africa, while decreasing in low-income or the least developed economies. In addition, the role of macroeconomic and institutional factors in explaining income inequality is limited and differ across convergence clubs. Evidence suggests the importance of fiscal, employment and monetary policies and the rule of law to tackle inequality in high-income economies, while they have no statistically significant role in low-income economies’ income inequality

Delocalization of edge states in topological phases

The presence of a topologically non-trivial discrete invariants implies the existence of gapless modes in finite samples, but it does not necessarily imply their localization. The disappearance of the indirect energy gap in the bulk generically leads to the absence of localized edge states. We illustrate this behavior in two fundamental lattice models on the single-particle level. By tuning a hopping parameter the indirect gap is closed while maintaining the topological properties. The inverse participation ratio is used to measure the degree of localization.Comment: 5 pages including 6 figures and 4 pages supplement including 8 figur

Polarimetric Modelling for Anisotropic Stars

A theoretical description of the polarization of light, generated by an anisotropic point light source and scattered by an arbitrary shape envelope, is developed in this work, the mechanism of scattering being assumed to be either Thomson or Rayleigh scattering. The description is a development of the earlier work of J. F. L. Simmons' (1982) where he expressed the scattering function and scatterer density distribution function as a summation of multipole contributions. Whereas Simmons analysis was based on an isotropic point light source, the present analysis permits a variable flux to represent the anisotropy of the light source. Thomson or Rayleigh scattering is assumed throughout, and in all cases the scattering envelope is taken to be large compared to the light source. This allows the anisotropy to be expressed in terms of projected area. The model has applicability to rotating, pulsating, binary, and active stars with hot extended envelopes. The thesis is divided into five chapters plus four appendices. Following a review of previous work in Chapter One, together with a discussion of the motivation and interest of stellar polarimetry, in Chapter Two the theoretical analysis is established for scattering polarization with an anisotropic point light source within a spherical envelope. This analysis is then applied to an ellipsoidal black body star within a spherical envelope, for which we get explicit integral expressions for the Stokes' parameters and an analytical solution for the special case of a star with a circular equator. As examples of ellipsoidal stars the polarization from a single distorted star (due to e. g. the rotation) such as Be stars, X-ray binaries filling its Roche lobe (e. g. Cygnus X-1 and Centaurus X-3) , and pulsating stars (pulsating as a series of ellipsoids) is calculated. The latter show a very complicated pattern of qu-loci, which, in principle, fit the polarization behaviour of such types as RV Tau and Omicron Ceti. The maximum polarization of about 20% of the total light is expected from a disk like light source viewed edge on (Galaxies would be good example, since they are very distorted light sources). In Chapter Three the anisotropic light source theory is generalized to include an arbitrarily shaped envelope. In the harmonic summation which results it is found that approximation up to the second order terms is quite acceptable, when both the light source and the envelope are ellipsoidal. The maximum polarization is enhanced (due to the envelope being ellipsoidal) to about 35% when a disk of scatterers is perpendicular to the disk like star observed edge on. In general whether the polarization undergoes enhancement or cancellation is dependent on the angle between the rotation axis of the ellipsoidal star and the axis of symmetry of the ellipsoidal envelope. The effects of rotation and pulsation are also calculated. In Chapter Four the analysis is applied to the case of light source anisotropy arising from a non-uniform photosphere (e. g. hot or cool spot). Calculation of the projected area of the spot as it varies during stellar rotation is done without any of the simplifying assumptions usually made in stellar light curve modelling. Again the approximation of the second order terms of the harmonic summation is acceptable for spots of likely physical size (e. g. of angular extent < 3